chore: update changelogs and bump to 2.7.0.dev (#21949)
* Set development version to 2.7.0.dev
* docs: record the 2.6.5 and 2.6.6 releases in the changelogs
Move the CVE-2026-58659 fixes out of pytorch's [unreleased] into a
[2.6.6] section, and add a fabric [2.6.5] section for #21595, which
landed on master but was only ever recorded on release/stable.